The skin serves numerous functions but its primary function is as a protective layer or barrier. The most important role of the skin for terrestrial animals is to protect the water-rich internal organs from the dry external environment. In addition, the skin protects internal tissues from harmful chemical and physical forces as well as from the penetration of pathogens.
Skin is a structurally complex thick membrane. The skin is composed of the epidermis, the dermis, the hypodermis, and the adenexal structures (epidermal appendages). The epidermis, the outermost epithelial tissue of the skin, is itself composed of several layers—the stratum corneum, the stratum granulosum, the stratum spinosum, and the stratum basale. The epidermis is mainly constructed of keratinocytes, and is in a constant state of self-replacement. At the bottom layer, keratinocyte stem cells divide into daughter cells that are displaced outward, and which differentiate through successive overlying layers to enter the stratum corneum. Then, the keratinocyte cells go through apoptosis and die, and their cellular organella and cytoplasm disappear during the final process of differentiation.
The water impermeability function of the skin resides in the upper thinnest layer (approximately 10-20μ in humans) of stratum corneum (SC), having about three magnitudes of order higher water resistance compared to other membranes of living organisms.
The hydration state of the stratum corneum is important for its barrier function as well as for the healthy and firm appearance of the skin. Stratum corneum hydration and transepidermal water loss (TEWL) are two key indexes used for its characterization, typically showing an inverse relationship. High TEWL values, as a marker of disturbed skin barrier function, are frequently correlated with low hydration of the Stratum corneum. Disrupted barrier functions can result inter alia, from skin cleansing with soaps and detergents or from various diseases of the skin. For example, in atopic dermatitis skin barrier disruption has been confirmed by electron microscopy and penetration studies, and lesional atopic dermatitis show high TEWL values. The mechanisms of this interplay of the stratum corneum hydration and transepidermal water loss have not been studied exhaustively. It is suggested however, that disturbed skin barrier function leads to changes in epidermal differentiation.
Tissue integrity is highly dependent on proper cell to cell adhesion, performed by Cell-to-Cell Junctions. Cell-To-Cell Junctions are complexes comprising transmembrane adhesive proteins, and their forms and distribution may vary according to the tissue type. In the skin the prominent junctions are desmosomes, tight and gap junctions.
Desmosomes are intercellular junctions that tightly link adjacent cells. The protein desmoplakin is an obligatory component of functional desmosomes, which anchors intermediate filaments to desmosomal plaques.
Tight junctions (TJ) are cell-cell junctions which connect neighboring cells, control the paracellular pathway of molecules (“barrier function”) and separate the apical part from the basolateral part of a cell membrane (“fence function”). The composition of the tight junctions seems to be very important for their barrier function in the epidermis. Down-regulation as well as overexpression of certain proteins perturbs this barrier. Damage to the tight junctions is linked to skin water loss, dry and sensitive skin and to several skin disorders, including psoriasis vulgaris, lichen planus, cholangitis and ichthyosis, including ichthyosis vulgaris.
Gap Junctions are organized collections of protein channels in cell membranes that allow ions and small molecules to pass between adjacent cells. The protein channels that make up gap junctions consist of two hexagonal arrays of membrane-spanning proteins termed connexons. One connexon resides in the membrane of one cell. It aligns and joins the connexon of the neighboring cell, forming a continuous aqueous pathway by which ions and small molecules can freely (passively) pass from one cell to the other.
Several synthetic compounds as well as plant extracts have been shown to affect the skin barrier function. For example, U.S. Pat. No. 8,101,214 discloses herbal extract complex of several Chinese herbs at a ratio of the plant extracts that is superior in promoting the differentiation of skin keratinocytes, restoring impaired skin barrier function and increasing skin moisturization as compared to extracts obtained by extracting each of the herbs alone.
U.S. Pat. No. 8,168,197 discloses a skin external composition for alleviating dry skin symptoms, which contains an extract of Scrophularia buergeriana Miq. as a main component and further contains an extract of Poria cocos Wolf, as well as the use thereof for skin-moisturizing cosmetics. The composition contains, as active ingredients, the Scrophularia buergeriana Miq. extract and the Poria cocos Wolf extract, which are prepared by extracting each separately, using water, ethanol, methanol, hexane, ethyl acetate or butanol.
U.S. Patent Application Publication No. 20040156818 discloses compositions comprising a blend of neem seed cell broth and one or more additional botanical ingredients or pomegranate fruit extract and, optionally, one or more additional botanical ingredients which are useful, among others, in reduction in skin fragility, improvement in skin barrier repair and/or function and improvement in skin moisturization.
International (PCT) Patent Application Publication No. WO 2008/148694 discloses a moisturizing cosmetic composition comprising a galacto-oligosaccharide that consists of 4 to 10 monosaccharide units and has at least one fructose residue. This galacto-oligosaccharide may be obtained from a plant belonging to the Lamiaceae family flora. An extract from this plant showed improved skin moisturizing effect without stickiness, and a synergetic moisturizing effect with polyhydric alcohols.
International (PCT) Patent Application Publication No. WO 2009/106934 discloses Angico-Branco (Piptadenia colubrina) extract useful as a skin barrier protective and moisturizing agent, due to mechanisms involving stimulation of the expression of aquaporin-3, fibronectin and the envelope proteins filaggrin and involucrin. The invention further relates to cosmetics and dermatological formulations comprising the extract for facial or body treatment of specific skin changes such as skin dryness, cracking, scaling, flaking or any disturbance involving skin barrier disruptions.
Exemplary list of plant extract based products useful in promoting skin barrier function, already available on the market include PhytoCellTec Alp Rose of Mibelle Industries, Switzerland and Phytoglycolipid II of Barnet Products, U.S.A.
The continuously growing list of products and technologies attempting to improve the skin barrier functions show that a universally useful solution is not yet available.
Jojoba (Simmondsia chinensis) is a shrub native to the Sonoran and Mojave deserts of Arizona, California, and Mexico. It is the sole species of the family Simmondsiaceae, placed in the order Caryophyllales. It is also known as goat nut, deer nut, pignut, wild hazel, quinine nut, coffeeberry, and gray box bush. Jojoba is grown commercially for its oil, a liquid wax ester present in the seeds. Jojoba oil is used as a replacement for whale oil and its derivatives, such as cetyl alcohol. Prohibiting the import of whale oil to the U.S. in 1971 led to the discovery that jojoba oil is in many regards superior for applications in the cosmetics and other industries. Jojoba oil is also a fungicide, and can be used for controlling mildew. Like olestra, jojoba oil is edible but non-caloric and non-digestible. Jojoba biodiesel has been explored as a cheap, sustainable fuel that can serve as a substitute for petroleum diesel.
Use of hydrophilic extracts of Jojoba has been previously disclosed only for whitening and exfoliating skin, utilizing the jojoba meal, particularly the meal retained after the oil extraction. The meal is mainly used as a mechanical peal in combination of one or more hydroxy acids (U.S. Pat. Nos. 6,890,566, 7,025,957, 7,029,709, 7,097,866).
As described hereinabove, promotion of barrier function and subsequent maintenance of skin integrity are important for improving skin appearance and health including treating and/or alleviating various skin disorders. It is highly desirable and it would be advantageous to have compositions and methods effective in promoting the barrier functions of the skin, particularly plant-derived compositions known not to have deleterious effects.